As you'll recall, function invocation with the
() operator is an expression. All
expressions have values, and the return
statement is used to specify the value "returned by" a function.
This value is the value of the function invocation expression.
The syntax of the return statement is:

return [ expression ];

When the return statement is executed, the
expression is evaluated, and returned
as the value of the function. Execution of the function stops when
the return statement is executed, even if there are other
statements still remaining in the function body. The
return statement can be used to return a value
like this:

function square(x) { return x*x; }

The return statement may also be used without
an expression to simply terminate
execution of the function without returning a value. For example:

function display_object(obj) {
// first make sure our argument is valid
// and skip rest of function if it is not.
if (obj == null) return;
// rest of the function goes here...
}

If a function executes a return statement with
no expression, or if it never executes
a return statement (i.e., it simply executes
all the statements in the body and implicitly returns) then the
value of the function call expression will be undefined (i.e., the
special JavaScript undefined value).

It is a syntax error to use the return
statement anywhere except in a function body.